SRAM cell with dynamic split ground and split wordline

ABSTRACT

An SRAM cell with dynamic split ground (GND) and split wordline (WL) for extreme scaling is disclosed. The memory cell includes a first access transistor enabled by a first wordline to control access to cross coupled inverters by a first bitline. The memory cell further includes a second access transistor enabled by a second wordline to control access to the cross coupled inverters by a second bitline. The memory cell further includes a split ground line comprising a first ground line (GNDL) separated from a second ground line (GNDR). The GNDL is connected to a transistor of a first inverter of the cross coupled inverters and the GNDR is connected to a first transistor of a second inverter of the cross coupled inverters.

FIELD OF THE INVENTION

The invention relates to semiconductor structures and, moreparticularly, to an SRAM cell with dynamic split ground (GND) and splitwordline (WL) for extreme scaling.

BACKGROUND

Due to the increased portion of Static Random Access Memory (SRAM)arrays in the total chip area of modern chips, device dimensions in SRAMmust be continuously scaled. However, SRAM stability degrades rapidlywith scaled devices. Various methods have been employed to maintainstability but generally at the cost of performance, density and designassist overhead. For example, one approach is to use asymmetric SRAMcells to improve stability with one-sided sensing. In this approach, thewordline is split into a wordline left (WL) and a wordline right (WR).The SRAM can be read on one side only with either the WR or the WL. Inthis way, during read operation only the word line of the sensing sideis activated.

Although stability can be enhanced with asymmetric designs, the SRAMcell is not useful for differential sensing. That is, the SRAM cell isnot useful for SRAM designs with standard differential sensing from abitline left (BL) and a bitline right (BR). Asymmetric designs alsoimpose undesirable constraints on the SRAM applications. For example,the bit line on one side cannot be used as a 2nd read port. Thiseffectively slows down the read/write operations. Also, the SRAM cellcannot serves as standard cell type where the asymmetric stability isnot needed. Power down scheme in low power applications gets morecomplicated or compromised.

SUMMARY

In an aspect of the invention, a memory cell comprises a first accesstransistor enabled by a first wordline to control access to crosscoupled inverters by a first bitline. The memory cell further comprisesa second access transistor enabled by a second wordline to controlaccess to the cross coupled inverters by a second bitline. The memorycell further comprises a split ground line comprising a first groundline (GNDL) separated from a second ground line (GNDR). The GNDL isconnected to a transistor of a first inverter of the cross coupledinverters and the GNDR is connected to a first transistor of a secondinverter of the cross coupled inverters.

In an aspect of the invention, a memory cell comprises: cross coupledinverters comprising PFETs and NFETs; a bitline left (BL) which accessesa first inverter of cross coupled inverters by enabling a first accesstransistor; a bitline right (BR) which accesses a second inverter of thecross coupled inverters by enabling a second access transistor; awordline left (WL) with enables the first access transistor; a wordlineright (WR) which enables the second access transistor; and a splitvertical ground line comprising a first vertical ground line (GNDL)separated from a second vertical ground line (GNDR), the GNDL isconnected to the first inverter of the cross coupled inverters and theGNDR is connected to the second inverter of the cross coupled inverters.

In an aspect of the invention, read operation of a memory cell comprisesenabling a first transistor of an asymmetric memory cell to allow accessof a first bitline to a storage cell, while raising or lowering avertical ground line of two separated ground lines connected totransistors of the storage cell above or below ground, GND.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is described in the detailed description whichfollows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the present invention.

FIG. 1 shows an SRAM cell in accordance with aspects of the presentinvention;

FIG. 2 shows a performance graph comparing the SRAM cell designs of thepresent invention to a conventional asymmetric SRAM;

FIG. 3 shows a table of extended exploitation of split GND for SRAMscaling; and

FIG. 4 shows the impact of the SRAM cell design of the present inventionto data retention margins (DRM).

DETAILED DESCRIPTION

The invention relates to semiconductor structures and, moreparticularly, to an SRAM cell with dynamic split ground (GND) and splitwordline (WL) for extreme scaling. In embodiments, the SRAM cell designincludes separate vertical SRAM GND buses, e.g., GND left (GNDL) and GNDright (GNDR). Advantageously, the SRAM cell of the present inventionthus provides increased stability gain for asymmetric devices, whilealso allowing for differential sensing.

In operation, GNDL and GNDR can be dynamically shifted to enhancestability of the asymmetric device, depending on the read side of thecell. For example, GNDL can be shifted upwards, e.g., increase voltageabove GND, while the sensing is on bitline right (BR); whereas, GNDR canbe shifted upwards, e.g., increase voltage above GND, while sensing ison the bitline left (BL). Also, advantageously, both BL and BR can servefor 1-sided sensing, besides serving for more effective write operation.Moreover, for less demanding SRAM arrays where stability assist is notneeded, setting GNDL=GNDR=0 allows standard SRAM operations such asdifferential sensing and global power down. Extended enhancement onwritability and power/performance also becomes available with differentGND shift patterns, implemented with the SRAM cell design of the presentinvention. Thus, in SRAM with fast NFET for quick access or with highbit line loading for array layout efficiency stability is enhanced withdynamic split GND.

The SRAM cells of the present invention can be manufactured in a numberof ways using a number of different tools. In general, though, themethodologies and tools are used to form structures with dimensions inthe micrometer and nanometer scale. The methodologies, i.e.,technologies, employed to manufacture the SRAM cells of the presentinvention have been adopted from integrated circuit (IC) technology. Forexample, the structures of the present invention are built on wafers andare realized in films of material patterned by photolithographicprocesses on the top of a wafer. In particular, the fabrication of theSRAM cells of the present invention uses three basic building blocks:(i) deposition of thin films of material on a substrate, (ii) applying apatterned mask on top of the films by photolithographic imaging, and(iii) etching the films selectively to the mask.

FIG. 1 shows an asymmetric SRAM cell in accordance with aspects of thepresent invention. In embodiments, the asymmetric SRAM cell 10 includesa bitline right BR and bitline left BL, in addition to a wordline rightWR and a wordline left WL. The asymmetric SRAM 10 further includes twocross-coupled inverters 15, comprising four transistors P0, P1, NO andN1; although more transistors per bit for multi-port applications arealso contemplated by the present invention. In this example embodiment,the transistors P0, P1 are PFETs and the transistors NO, N1 are NFETs.

The asymmetric SRAM 10 further includes access transistors T0 and T1.The access transistors T0 and T1 serve to control the access to thestorage cell, e.g., cross-coupled inverters 15, during read and writeoperations. For example, access to the storage cell 15 is enabled by theword lines WL and/or WR, which controls the respective accesstransistors T0 and T1 which, in turn, control whether the storage cell15 should be connected to (e.g., accessed by) the respective bit linesBL and BR. Thus, as should be understood by those of skill in the art,the access transistors T0 and T1 are used to transfer data for both readand write operations.

CL is the node across T0 from BL and CR is the node across T1 from BR.During global reset of memory array to zero state, the GNDL is raised toVdd to pull down CR and push up CL. During global set of memory array toone state, the GNDR is raised to Vdd to pull down the CL and push up theCR. During global set/reset of specific columns of memory array tospecific zero state and one state, the GNDL of specific columns areraised to Vdd, and the GNDR of other specific columns are raised to Vdd.

The asymmetric SRAM 10 further includes a vertical split ground line,e.g., GNDL and GNDR. In embodiments, the ground line GNDL is connecteddirectly to transistor NO on the left side of the asymmetric SRAM 10;that is, the ground line GNDL is isolated from transistor N1 on theright side of the asymmetric SRAM 10. In contrast, the ground line GNDRis connected directly to transistor N1 on the right side of theasymmetric SRAM 10; that is, the ground line GNDR is isolated fromtransistor NO on the left side of the asymmetric SRAM 10. Thus, thevertical GND lines (GNDR and GNDL) are kept separate; instead of beingstrapped together as in conventional SRAM cells.

In embodiments, the SRAM cell 10 has three different operating modes orstates: standby mode, reading mode and writing mode. In standby mode,the SRAM cell 10 is idle; whereas, in reading mode, the data are readout and in writing mode, the contents of the cell are updated. Byimplementing the designs of the present invention, e.g., vertical splitground lines GNDL and GNDR, the SRAM cell 10 of the present inventionprovides the necessary “stability” and “write-ability” which had beendiminishing in conventional SRAM cells of scaled down dimensions.

In operation during read access for bitline BL, the wordline WL israised to bring down the bitline BL for sensing. In one embodiment, theGNDL is lowered by about 10% of Vdd below common ground, GND. In furtherembodiment, GNDR can be simultaneously raised above common ground GND byabout 10% of Vdd to minimize read disturb.

Similarly, in operation during read access for bitline BR, the wordlineWR is raised to bring down the bitline BR. Also, GNDR is lowered belowcommon GND to help sensing. In further embodiment, the GNDL can besimultaneously raised by about 10% of Vdd above common GND to minimizeread disturb.

In SRAM cells with less demand on stability, GNDL and GNDR are keptcommon, e.g., GNDL=GNDR=0, as in conventional designs with standarddifferential sensing and bit column control circuitry. Accordingly, inthis way the asymmetric design of the SRAM cell 10 allows fordifferential signaling, which makes small voltage swings more easilydetectable. Also, in embodiments, two (2) read port operations with GNDshift assist are still available if the same row of cells is notaccessed in the same cycle or if the access assist for reading orwriting of one side does not ruin or degrade the access of the otherside.

In split GND schemes as contemplated by the present invention, furtherimprovement of margins, performance, and power are accomplished withdifferent combinations of GNDL/GNDR shifts. For example, FIG. 2 shows agraph of stability of the SRAM cell designs with a shift of GNDL. Inthis graph, the y-axis represents sigma (of Access Disturb Margin (ADM))and the x-axis is supply voltage. As shown in this graph, whenGNDL=GNDR=0, the sigma is about 4.0, which is below the requiredstability at 6.0 σ (sigma) for a possible failure rate around 1 part perbillion. However, when GNDL is about 10% of Vdd (Vcs) above GND, thestability of the cell increases to above 6 σ for low Vdd applicationaround 0.61V. Accordingly, even a GND shift of approximately 10% Vddwill improve Access Disturb Margin (ADM) by approximately 2.0 σ. Thescaled down SRAM cells to be discarded for poor operation margins canthus be revived with robust operation margins by the split GNDarrangement. ADM further increases when GNDL is raised with 15%-20% ofVdd above common ground GND before hitting the lower bound of thetransient DRM (data retention margin) of all cells along the selectedcolumns. With a split GND assist design (e.g., GNDL, GNDR), ADMdegradation from extreme scaling no longer constraints SRAM applicationswhen split GNDL or GNDR voltage can be independently adjusted around thecommon ground GND.

FIG. 3 shows a table of extended exploitation of split GND for SRAMscaling. As shown in FIG. 3, the split GND design of the presentinvention provides very efficient write assist when the cell GNDL orGNDR is elevated above the bit line down level 0 (referred as elevatedGND assist). There is further power saving when the standby cell Vdd isat an elevated GND (e.g., GNDH). In this representation, GNDH iselevated to about 10-15% of Vdd above GND. Also, as shown in the tableof FIG. 3, stability assist and write assist are provided with smallvoltage shift of GNDL or GNDR relative to common GND. Write0 refers topulling down CR and pushing up CL, where CR is the cell node on theright side and CL is the cell node on the left side. Write operation isassumed to be done with complementary input on BL and BR from activationof both WL and WR. Single port write is also made feasible withcomplementary GND shift of GNDL and GNDR in this split GND construct.Column GND shift assist would be more convenient if it can be applied towhole row of cells for simpler circuitry of column select.

FIG. 4 shows a table of the impact of the split GND design of thepresent invention on DRM (Data Retention Margin). As shown in FIG. 4,there is negligible impact of DRM on a cell within the same column whenGNDR GNDL when the operation voltage is around 0.6 V or higher.

The method(s) as described above is used in the fabrication ofintegrated circuit chips with SRAM cells. The resulting integratedcircuit chips can be distributed by the fabricator in raw wafer form(that is, as a single wafer that has multiple unpackaged chips), as abare die, or in a packaged form. In the latter case the chip is mountedin a single chip package (such as a plastic carrier, with leads that areaffixed to a motherboard or other higher level carrier) or in amultichip package (such as a ceramic carrier that has either or bothsurface interconnections or buried interconnections). In any case thechip is then integrated with other chips, discrete circuit elements,and/or other signal processing devices as part of either (a) anintermediate product, such as a motherboard, or (b) an end product. Theend product can be any product that includes integrated circuit chips,ranging from toys and other low-end applications to advanced computerproducts having a display, a keyboard or other input device, and acentral processor.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed:
 1. A memory cell, comprising: cross coupled inverters;a bitline left (BL) which accesses a first inverter of the cross coupledinverters; a bitline right (BR) which accesses a second inverter of thecross coupled inverters; a wordline left (WL) which enables a firstaccess transistor; a wordline right (WR) which enables a second accesstransistor; and a split vertical ground line comprising a first verticalground line (GNDL) separated from a second vertical ground line (GNDR),the GNDL being connected to the first inverter of the cross coupledinverters and the GNDR being connected to the second inverter of thecross coupled inverters, wherein: the GNDL and the GNDR are separatevertical SRAM GND buses, and in a standby mode of the memory cell, Vddis at an elevated GND (GNDH).
 2. The memory cell of claim 1, wherein theGNDH is elevated above GND by 10-15% of the Vdd.
 3. The memory cell ofclaim 1, wherein in the standby mode, the memory cell is idle.
 4. Thememory cell of claim 1, wherein the GNDR and the GNDL are not strappedtogether.
 5. The memory cell of claim 1, wherein the GNDR and GNDL allowfor differential signaling.
 6. The memory cell of claim 1, wherein: theGNDL is connected directly to an NFET of the first inverter; and theGNDR is connected directly to an NFET of the second inverter.
 7. Thememory cell of claim 1, further comprising: a left side cell node (CL)across the first access transistor the BL; and a right side cell node(CR) across the second access transistor from the BR, wherein: duringglobal reset of memory array to zero state, the GNDL is raised to theVdd to pull down the CR and push up the CL; during global set of memoryarray to one state, the GNDR is raised to the Vdd to pull down the CLand push up the CR; and during global set/reset of specific columns ofmemory array to specific zero state and one state, the GNDL of specificcolumns are raised to the Vdd, and the GNDR of other specific columnsare raised to the Vdd.
 8. The memory cell of claim 1, wherein the crosscoupled inverters comprise PFETs and NFETs.
 9. The memory cell of claim8, wherein during read access for the first bitline, the GNDR is raisedabove GND by about 10% of the Vdd and/or the GNDL is lowered below theGND by about 10% of the Vdd.
 10. The memory cell of claim 8, whereinduring read access for the second bitline, the GNDL is raised above GNDby about 10% of the Vdd and/or the GNDR is lowered below the GND byabout 10% of the Vdd.
 11. The memory cell of claim 1, wherein: the BLaccesses the first inverter of the cross coupled inverters by enablingthe first access transistor, and the BR accesses the second inverter ofthe cross coupled inverters by enabling the second access transistor.12. The memory cell of claim 1, wherein the first access transistor andthe second access transistor are NFETs.
 13. The memory cell of claim 12,wherein the first inverter and the second inverter further comprisePFETs.
 14. The memory cell of claim 1, wherein the GNDR and GNDL arekept common to allow for differential signaling.
 15. The memory cell ofclaim 1, wherein voltage of the GNDR and voltage of the GNDL areindependently adjustable around a common GND.